Line balance for loaded telephone circuits



June 24, 1930; A, c. BARTL T'T 1,767,200

LI NB BALANCE FOR LOADED TELEPHONE CIRCUITS Original Filed March so, 1925 /NVENTOR ALBERT CHARL- 5 BA TLETT ATT Y.

Patented June 24, 1 930 PATEL ALBERT CHARLES BARTLETT, OF 'WATFORCD, ENGLAND, ASSIGNOR TO THE GENERAL I ELECTRIC COMPANY LIMITED, OF MAGNET HOUSE, LONDON, ENGLAND LINE BALANCE ron' LOADED TELEPHONE cracorrs Original application filed March 30, 1925, Serial No. 19,421,La1'1d in Great Britain April 4, 1924. Divided and this application filed March 25,-1927. Serial No. 178,458.

This invention relates to line balances for loaded telephone'circuits. The problem of making satisfactory line balances is that of devising some comparatively simple and cheap combination of impedance elements which shall have at all telephonic frequencies an impedance nearly the same as the sending end impedance of the actual line.

The object of the invention is to provide improved line balances the impedance of which can be made to simulate closely over the range'of telephonic frequencies the impedance of a periodically, loaded cable which is terminated at half cable section or over a considerable range of fractional cable section terminations. By a cable section is meant the length of cable between two loading coils.

- to be balanced, as hereinafter more;

g It has been shown in In the accompanying drawing, Fig. 1 shows oneform of the invention, and Figs. 2 and 3 show modifications.

A line balance according to the present invent-ion consists of a new arrangement of elements having inductance, capacity and resistance, the values of which may be determined in terms of the constants of the line I fully described. Although the line balance described in this specification is primarily a mid-cable sect-ion balance, yet it can be adapted to balance the line over a considerable range of fractional cable section terminations;

patent application Serial No. 19,421, filed March 30,1925, and of which this is a divisional application, that the network shown in Fig. 1 may be-arranged sothat over the useful telephonic range the net work gives a non-reactive resistance .decreasing with frequency in a desired manner, for let us consider the impedance of a periodically loaded cable, and let Z =Impedance of an infinite periodicallyv middle loaded line starting'from the point of a loading coil.

In practice both n=Frequency of the current in the line.

H =Inductance of each of the loading coils.

Z=Distance between loadin coils.- 0=Capacity per unit lengt of line. L=Inductance per unit length of line. R=Resistance per unit length of line. I1 l Then if the inductance of the line apart from the loading coils be negligible it is known that i JLLEZLE OZ 4 100 H than H 1) 1 jR 1 C Z TIE' TO F Kl v therefore one element of the line balance will R v and are always less creasing with frequency according to the re lation shown in equation (2), and is of zero rea'ctance the values of the elements being definitely related to the electrical constants of the line,.and being determined in the following manner p The impedance of the network of Fig. 1 is given by V 55 have thevalues .9 and" .6 at the same frequeneies. For the frequency at which 1 where The real part of Z that is the resistance component of the impedance of this network, is

' r 1 aK r 2KL2)'+ 1 mm 0 which is of the form Now the resistance component of the cable impedance from Equation (2) is seen to be 1 H07 OZ 4 which for simplicity can be written all- 1 7) B g-,5 and'A if By making 1'=B and choosing K and L of suitable values these expressions (6) and (7) can be made to have very nearly equal near the. two frequencies mentioned above. For example for .a medium loaded cable the values of are about 0.9 and 0.6.

The problem thenis tomake 1 up b1) and has the value .9

Thus we must have 1 .1911 0.19 2 1 FT 0. 9 .1'71a OI0325b A A2 and for the other value in a similar way Solving these equations for a and b b =A X 1.01 and a=A .396.

Thus d K r 2KL .396A K L ==1.-.0 1A Thus KL =A (approximately) Kr =2.3 96A. Lg. 1.55 A i K Thus 11 K and 1' are now all determined in terms of constants of the cable.

The reactance of the network of Fig. I is. from (4) y L p( 2(- P 2) 1 (1 p KL +12 K r Substituting values from above in the second term of this expression, the term becomes (0.905r /Z+0.6451-A 1+0.4p A +pA In practice this is approximately equal to .0.905 .r.JZ

' and therefore represents a negative in ductance. I

Accordingly if'we make the re'actance of the network of-Fig. 1 will be zero, and weshall have a network which over active resistance decreasing with frequency in the desired manner, the value ofeach element being determined by constants of the,

cable in accordance with the following table:'-

I Z 1=0.4? H

:=0.32H K =0;770l

' I z I Now if the impedanceof a eriodically loaded cable terminated at hal cable secthe useful telephonic range gives a non-retion be considered a closely approximate expression for the impedance is as 7 E P This can be expanded into the form 2 yr L 2 a PH t The second term can be closely represented by a series condenser of value a 2 VHUZ v while the first term representsa resistance. increasing with frequency of the form 1/ 1 where B and A have the same significance as above.

It has already beenshown that a-network of the form shown in Fig. 1 can be made to If, now, the network be constructed as shown in Fig. 2 such that and ' L r K it can be demonstrated easily that the product of its impedance and the impedance of the network shown in lis equal to 7' which equals B Thus since this latter network closely represents the resistance B /1-Ap 2 the required network must closely represent the re- I sistance ly obtained we have The mid-cable balance consists of *the net work of Fig. 2 having the yalues stated, in

series with a capacity of value k and the complete balance is shown in Fig. 3.

Since the short length of cable betweenv I consecutive loading coils can be represented closely by a shunt capacity OZ it follows that by changing the value k the balance can be made to simulate the cable approximately for any termination between full cable and (0.5-0.45) '=.05 cable termination, in which case [c -=0.

.1. A line balance, the impedance of which over the range of telephonic frequencies simulates closely the, sendin end impedance of a periodically loaded cine terminated at half cable section, comprisinga number of elements including one capacity arranged in parallel with a group of elements comprehending an inductance .in series with a resistance and a second capacity connected in parallel with the resistance and a third capacity'placed in series with the parallel circuits afforded by the one capacity and the group of elements, the values of-the several elements being determined in terms of the constants of the line to be balanced, sub;

stantially as described.

2. Aline balance, the impedance of which over the range of telephonic frequencies simulates closely the sending end impedance of a periodically loaded cable terminated at a fractional cable section termination comprising-a number of elements including one ca acity adjusted to the desired fractional ca le termination, and arranged in parallel with a group of elements comprising an inductance in series with a resistance and a second capacity connected in parallel wv'iththe resistance, and a third capacity placed 'in series with the parallel circuits afiorded by the one capacity and the group of elements, the values of the several elements of the balance first bein determined in terms of the constants of t e line to be balanced, substantially as described.

3. A linebalance the impedance of which over the range of telephonic frequencies simulates closely the sendin end impedance of a periodically loaded ca le terminated at a small fractional cable section termination,

comprising a number of elementsincluding one capacity adjusted to a value correspond-' ing with the desired fractional cable section termination said value being included within a range of values havin thereof, the said capacity% ing an inductance in series with a resistance and a second capacity connected in parallel with the resistance, and a third capacity conzero as one limit eing. arranged in parallel with a group of elements comprisnected in series with the paraillel circuits afiorded by the one capacity and the group of e1ements,the values of the several ele-' ments of the balancefirst being determined in terms of the constants of the line to be balanced.

In testimony whereof I affix my signature.

ALBERT CHARLES BARTLETT." 

